Kenneth Holmberg VTT Technical Research Centre of Finland

Kenneth Holmberg

VTT Technical Research Centre of Finland

OECD IRG 26th Meeting, Lyon, France, 5-6.10.2006

REPORTING EXPERIMENTAL RESULTS IN TRIBOLOGY -

SUMMARY OF THE DISCUSSION ON NEW WEAR RATE UNIT

VTT TECHNICAL RESEARCH CENTRE OF FINLAND

BACKGROUND

• Important role of tribology in our society: safety, reliability, energy, new products

• Tribology develops and expands: number of meetings, conferences, articles submitted, published

• New and more generic and precise theories: needs to be confirmed empirically

• How good is our reported experimental data?


BACKGROUND

• Experience from writing a tribology articles and books

• Many articles are today published that are never or seldom referred to - what is their contribution to our general knowledge?

• In tribology we are presenting scatters of experimental results that would not be acceptable in other fields of natural science!

• Two basic problems:

1. Specifying experimental conditions

2. Reporting wear


1. SPECIFYING EXPERIMENTAL CONDITIONS

• 25th IRG OECD meeting in Uppsala, June 2005, decided to recommend a short list of parameters with which a tribological contact can be described


BASIC TRIBOEMPIRIC PARAMETERS

1. Energy input: speed, load

2. Materials for body 1 and 2: • Physical / chemical / metallurgical specification• H, E, Ra• Coating: h, H, E + phys/chem/met specification

3. Third body:• Lubricant: η + chemical composition• Contaminants: material, size, H

4. Environment:• Humidity, temperature


IRG OECD RECOMMENDATION OF MINIMUM SPECIFICATIONS FOR EXPERIMENTAL CONDITIONS IN TRIBOLOGY ARTICLES

Description of test device

- a brief description of the test system or in case of commercially available device, its code.

Geometry

For both surfaces in contact:

- macrogeometry: given typically as radius or diameter of a curved surface, or contact spot dimensions for flat contacts, so that stresses can be determined from loads,

- microgeometry: surface roughness, given as appropriate for the application, but also as Ra-value for comparison.

Energy input

- load (or stress),

- speed.

Materials

For both surfaces in contact:

- material specification: metallurgical, physical or chemical; including processing method when appropriate,

- any coating material, coating thickness and deposition or surface treatment method,

- hardness and Young's modulus,

- fracture toughness, when possible.

For abrasion and erosion

- abrasive or erodent material, size, size distribution and shape,

- speed and direction of fluid stream for erosion.

Environment

- lubricant: viscosity and chemical composition/specification of fluid and additives,

- humidity,

- temperature,

- possible gases, radiation etc.

Results

- friction as coefficient of friction; typically the steady state value and the maximum value,

- wear for both surfaces as:

-- wear rate (e.g. 10^-6 mm3/N·m) and/or

-- wear volume per time or per sliding distance and/or

-- wear depth per time or per sliding distance,

- for fatigue wear the endurance life time,

- graphs of friction and wear versus time when appropriate,

- surface observations of both surfaces in contact, including observations of surface layers and their

extent,

- observations of wear debris and when possible their average size and size distribution.


1. SPECIFYING EXPERIMENTAL CONDITIONS

• 25th IRG OECD meeting in Uppsala, June 2005, decided to recommend a short list of parameters with which a tribological contact can be described

• The list has been distributed to 15 tribology journals

• Response positive - already in use in Tribotest and Journal of Engineering Tribology, IMechE


TRIBOLOGY JOURNALS APPROACHED

Wear

Tribology International

Tribology Transactions - STLE

Tribology Lubrication Technology - STLE

Journal of Engineering Tribology, Part J - IMechE

Industrial Lubrication and Tribology

Tribology Letters

Tribotest

Tribologia - Finnish Journal of Tribology

Tribologia - Teoria i Praktyka

Tribologie und Schmierungtechnik

Surface and Coatings Technology

Thin Solid Films

Surface Engineering

Trenie I Iznos


2. REPORTING WEAR

• Comparison of tribological performance often done based on Friction even if Wear is more important!

• Because μ is a good uniform parameter.• Wear is reported in many forms:

• wear volume• wear depth• wear rate/factor/coefficient/constant….• wear volume/time or distance or speed…..• etc


WEAR PARAMETERS

• We need a more uniform way to express wear.• What is that?• Czichos suggested Wear Rate ( )

where k = W / (s · F)• k = wear volume / input of energy into the contact. If

hardness is included there is a loss of generality.• Sometimes we need W = wear volume, e.g. erosion• Sometimes we need h = wear depth, e.g. tolerances• Sometimes we need t = endurance life, e.g. fatigue• Do we need more wear parameters?• Should we use k, or COW similar to COF, or the Greek

κ similar to μ?

10-6 · mm3 / m · N


REPORTING WEAR IN THE LITERATUREParameters, symbols and units used in addition to wear volume, wear depth and wear mass. Parameter Symbol Unit Reference Wear coefficient - 10-7· mm3 / (N·m) Abren et al., Wear 259(2005)771-778

Wear coefficient k - Bhushan, Principles and Applications of Tribology, John Wiley, 1999, p.484

Wear coefficient K - Suh, Tribophysics, Prentice Hall, 1986, p.13

Wear coefficient k - Hsu & Shen, in Modern Tribology Handbook (Ed. Bhushan) 2001, Vol.1,p.323

Wear coefficient k - Dunaevsky, in Tribology Data Hanbook (Ed. Booser) 1997, p.456

Wear coefficient K - Kato, in Tribology of Mechanical Systems (Eds Vizintin et al., 2004, p.192

Wear coefficient K - Hsu et al., J. Engeneering Tribology, IMechE, 216(2002)428

Coefficient of wear k 10-6· mm3 / (N·m) Klafke et al., Tribotest 11(2005)213-232

Coefficient of wear K - Kato & Adachi, in Modern Tribology Handbook (Ed. Bhushan) 2001, Vol.1, p.279

Specific wear rate - mm3 / (N·m) Ramalho et al., Wear259(2005)828-834

Specific wear rate - 10-5· m3 / (N·m) Biswas, J. Engeneering Tribology, IMechE, 216(2002)361

Specific wear rate - 10-6· mm3 / (N·m) Friedrich et al., J. Engeneering Tribology, IMechE, 216(2002)417

Wear rate - m3 / (N·m) Barros'Bouchet et al., Trib. Int.38(2005)257-264

Wear rate K 10-6· mm3 / (N·m) Holmberg & Matthews, Coatings Tribology, Elsevier, 1994, p.53

Wear rate K - Dorinson & Ludema, Mechanics and Chemistry in Lubrication, Elsevier, 1985, p. 357 & 362

Wear rate k mm3 / s Hsu & Shen, in Modern Tribology Handbook (Ed. Bhushan) 2001, Vol.1,p.326

Wear rate - mm3 / m Ruff, in Tribology Data Hanbook (Ed. Booser) 1997, p.436

Wear rate K 10-15· m3 / (N·m) Blanchet, in Tribology Data Hanbook (Ed. Booser) 1997, p.550

Wear rate - 10-12· m3 / m Biswas, J. Engeneering Tribology, IMechE, 216(2002)363

Wear rate - mm3 / (N·m) Miyoshi, Solid Lubrication, Marcel Dekker, 2001, p.262

Specific wear amount Ws mm3 / (N·m) Kato, in Tribology of Mechanical Systems (Eds Vizintin et al., 2004, p.191

Wear constant - mm3 / m / N Ruff, in Modern Tribology Handbook (Ed. Bhushan) 2001,Vol.1,p.526

Wear factor - mm3 / (N·m) Ruff, in Tribology Data Hanbook (Ed. Booser) 1997, p.436

Hard coating wear - mm3 / m Gangopadhyay, in Tribology Data Hanbook (Ed. Booser) 1997, p.595


FIGURES/TRIBOLOGY/IRG Wear Rate & Friction-01-290605

Fn v Fμ

Friction = a force resisting motion

Wear = a process resulting in avolume of detached material

Fμ = μ · Fn

=> μ = Fμ / Fn

W = κ · Fn · s

=> κ = W / ( Fn · s )

• μ at a certain moment t1

• κ at a certain time interval t0 - t1 ~ s1

s1

t0 t1

COEFFICIENT OF FRICTION AND WEAR RATE


IRG OECD UPPSALA MEETING, 2005

• Recommendation from IRG OECD meeting with about 30 participants to introduce a new unit for wear rate: Bowden (B) equal to 10-6 · mm3 / (N · m)


EXAMPLES OF WEAR RATE UNITS

Comparison by examples of the use of the wear rate units (10-6 mm3/Nm), (10x mm3/Nm) and Bowden (B) Wear rate expressed with 10-6 mm3/Nm Wear rate expressed with 10-x mm3/Nm Wear rate expressed with B Wear rates of about 100 10-6 mm3/Nm were Wear rates of about 10-4 mm3/Nm were Wear rates of about 100 B were obtained...... obtained and ...... obtained and ...... ....very low coefficients of friction of 0.01 ....very low coefficients of friction of 0.01 ....very low coefficients of friction of 0.01 to 0.04, wear rates of only 0.001 10-6 to 0.04, wear rates of only 10-9 mm3/Nm to 0.04, wear rates of only 1 mB and excellent mm3/Nm and excellent film endurances........ and excellent film endurances........ film endurances....... However, the ball wear rate was slightly However, the ball wear rate was slightly However, the ball wear rate was slightly decreasing with increasing humidity, from decreasing with increasing humidity, from decreasing with increasing humidity, from 0.00012 to 0.00006 10-6 mm3/Nm while the 12 to 6 10-11 mm3/Nm while the coating wear rate 120 to 60 µB, while the coating wear rate coating wear rate increased remarkably from increased remarkably from 0.1 to 1.6 10-6 increased remarkably from 0.1 to 1.6 B 0.1 to 1.6 10-6 mm3/Nm with increasing mm3/Nm with increasing humidity in....... with increasing humidity in........ humidity in.......


IRG OECD UPPSALA MEETING, 2005

• Recommendation from IRG OECD meeting with about 30 participants to introduce a new unit for wear rate: Bowden (B) equal to

• Web: http://virtual.vtt.fi/virtual/proj3/irg/discussion.htm• Resulted in intensive discussion on the web with 27

contributions of which:• 13 are positive• 2 are neutral• 4 are negative• 12 come with new suggestions

• The suggestions were discussed in the "Cutting edge" column in Tribology & Lubrication Technology by Tysoe and Spencer

10-6 · mm3 / (N · m)


NEW SUGGESTIONS

• Other unit name suggestions: Archard (Ar), WEARON (WR or WE), Tabor (Ta), Czichos (Cz)

• There is a "Tabor parameter" in use to interpret the ratio of elastic deformation of two contacting bodies

• The wear unit should be expressed as: 10^-6mm^3/N-m

• The wear index (WI) should be used: WI = Wear volume • 10^6 GPa / (Normal load • Sliding distance)

• Use WIR for rotational wear, WIL for straight-line wear, WIF for fretting wear etc

• Use wear resistance Rabinowicz: 1 R = 1/B = 10^6 N·m/mm^3 = 10^-3 Pa


SCEPTIC COMMENTS

• Bowden not right name - he was more working on friction

• Introducing a new unit is confusing - no more derived units are needed

• The new unit will most likely not be adopte widely• The unit would add false validity to the Archard

equation - wear rate is totally system dependent• Pascal (Pa) is N/m^2 - thus will B = 1/Pa• Fatigue, erosive and impact wear cannot use Bowden• There are >30 important variables and ~70 minor ones

influencing on wear• Misleading that wear is more exact subject than it is -

wear is a statistically varying entity


CONTRIBUTORS TO THE DISCUSSION

• Philippe Kapsa, ECL, France

• Bo Jacobson, Lund Univ., Sweden

• Kenneth Holmberg, VTT, Finland

• Sture Hogmark, Uppsala Univ., Sweden

• Ian Hutchings, Cambridge Univ., UK

• Steve Shaffer, Battelle Memorial Inst., USA

• Staffan Jacobson, Uppsala Univ., Sweden

• Koji Kato, Sendai Univ., Japan

• Brian Briscoe, Imperial Col., UK

• Nic Spenser, Fed. Inst. Techn. Zurich, Switzerland

• Eddy Tysoe, Univ. Wisconsin-Milwaukee, USA

• Ken Ludema, Univ. Michigan, USA

• Michel Maillat, Lab. Dubois, Switzerland

• Steven Franklin, Philips, The Netherlands

• Yoshi Kimura, Japan

• James Kauzlarich, Univ. Virginia, USA

• Jordan Liu, Advanced Materials Technology, USA

• Per Kjeldsteen, Danfoss, Denmark


THREE SUGGESTIONS FOREXPRESSING WEAR:

1. The IRG OECD Uppsala suggestion:Wear Rate expressed with a new unit

Bowden (B) equal to

2. The earlier " Czichos " suggestion:Wear Rate

3. Wear resistance Rabinowicz: 1 R = 10^6 N·m/mm^3

10-6 · mm3 / (N · m)

10-6 · mm3 / (N · m)


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Kenneth Holmberg VTT Technical Research Centre of Finland